goo Ranking is a large Japanese polling site and in November 2007 over 1k people completed a questionnaire about what they thought an otaku looks like. Here is the top five results for both males and females, the source has the full list.

Q1: What characteristic in males makes you feel he looks like an otaku? (Sample size=1,101)

Anime print t-shirt

Anime print paper bag

Bum Bag/fanny pack

Wearing a bandanna around the head in a hachimaki style

Bag plastered with anime-related patches, stickers –

Q2: What characteristic in females makes you feel she looks like an otaku? (Sample size=1,101)